A substrate for a printed circuit board, the substrate including a metal layer stacked on a surface of an insulating base film composed of, for example, a resin, has been widely used, the substrate being to be formed into a printed circuit board by etching the metal layer to form a conductive pattern.
In recent years, with a trend toward miniaturization and higher performance of electronic appliances, there has been a demand for higher-density printed circuit boards. As a substrate for a printed circuit board that meets the demand for higher density, a substrate for a printed circuit board has been required to include a metal layer with smaller thickness.
Furthermore, a substrate for a printed circuit board, the substrate having high peel strength between a base film and a metal layer, has been required in order not to allow the metal layer to be peeled from the base film when a bending force is applied to the printed circuit board.
To deal with these demands, a substrate for a printed circuit board, the substrate including a thin copper layer stacked on a heat-resistant insulating base film without an adhesive layer, has been reported (see Japanese Patent No. 3570802). In the traditional substrate for a printed circuit board, the thin copper layer (first conductive layer) having a thickness of 0.25 to 0.30 μm is formed on each surface of the heat-resistant insulating base film by a sputtering method, and a thick copper layer (second conductive layer) is formed thereon by an electrolytic plating method.